added clock frequency test for atusb and updated documentation

- prod/atusb: added clock frequency test
- prod/doc/setup.hmac: added GSL (libgsl) build dependency
- prod/doc/setup.hmac: recommend use of NTP on the PC
- prod/doc/test.hmac: added atusb to the clock frequency section

prod/atusb

@@ -4,6 +4,7 @@
 
 
 USB_ID=20b7:1540
+CLOCK_DRIFT_PPM=50
 
 
 enumerate()
@@ -31,7 +32,8 @@ gpio_usb()
 xtal_usb()
 {
 	step "Crystal frequency"
-	todo
+	cmd "$LOCAL_EXEC atrf-xtal -d $LOCAL_DUT -p $CLOCK_DRIFT_PPM"
+	expect_re ppm   # we catch any problems via the exit code
 }
 
 

prod/doc/setup.hmac

@@ -43,7 +43,7 @@ and the test scripts have to be installed on the PC. For flashing the
 firmware, also dfu-util and the firmware binary are required.
 <P>
 The instructions below assume that a complete development environment
-is already present on the PC, including SDL and SDL_gfx.
+is already present on the PC, including SDL, SDL_gfx, and GSL.
 If any prerequisites are not met,
 compilation will fail, and you'll have to install the corresponding
 package and try again.
@@ -111,6 +111,21 @@ it from source.
 <!-- ---------------------------------------------------------------------- -->
 
 
+<SUBSECTION title="Install and run NTP">
+
+The clock frequency test for <B>atusb</B> measures the clock frequency
+relative to the PC clock. Many PC clocks are highly inaccurate, which
+would render the result of the measurement meaningless. To improve the
+quality of the reference clock, it should be disciplined by NTP.
+<P>
+If NTP is not available, it may be necessary to increase the test's
+tolerance by changing the variable <SAMP>CLOCK_DRIFT_PPM</SAMP> in
+the script <SAMP>atusb</SAMP>.
+
+
+<!-- ---------------------------------------------------------------------- -->
+
+
 <SUBSECTION title="Install firmware binary">
 
 The binary of the application running on <B>atusb</B> is transferred from

prod/doc/test.hmac

@@ -73,7 +73,7 @@ for an <B>atusb</B> DUT. It performs the following steps:
   <LI>LED (<B>atusb</B> only; not yet implemented)
   <LI>GPIO scan (not yet implemented)
   <LI>Identification
-  <LI>Crystal frequency (only implemented on <B>atben</B>)
+  <LI>Crystal frequency
   <LI>Spectrum
   <LI>Receive
   <LI>Send
@@ -137,12 +137,14 @@ eventually provide more detailed results for problems with the SPI interface.
 <!-- ---------------------------------------------------------------------- -->
 
 
-<SUBSECTION title="Crystal frequency (atben only)">
+<SUBSECTION title="Crystal frequency">
 
 This test measures the frequency of the crystal oscillator in the DUT.
 On <B>atben</B>, it does this by transmitting packets, and measuring
 the time between the SLP_TR pulse that starts the transmission and the
 interrupt signaling the end of the transmission.
+On <B>atusb</B>, the microcontroller counts the clock cycles and the
+number of cycles is compared with the PC's NTP-disciplined clock.
 <P>
 If this test fails, this may indicate that the load capacitors of the
 crystal are missing, badly soldered, or have the wrong value. It could